write-api: implement POST /api/people/:slug/account-level (administrator-only) #33

Deferred from write-api (PR #29). The plan listed account-level changes under People mutations but didn't ship a dedicated endpoint — the spec at specs/api/people.md calls for POST /api/people/:slug/account-level (administrator-only).

Required pieces:

• New route + write service
• Auth: requireAuth('administrator') (NOT 'staff | administrator' — only admin can elevate/demote)
• Audit trail in the commit message: Action: account-level.change, with Previous-Account-Level + New-Account-Level trailers
• Validation: cannot demote yourself if you're the only administrator (would lock everyone out)
• Tests: happy path; non-admin caller → 403; self-demotion last-admin → 422